The Wikipedia page on Eternal September, or the dumbing down of the Internet, is interesting in that it doesn't mention the Web. In summary, AOL allowed it's users access to Usenet starting September 2003 meaning there was a steady stream of neophytes on the 'net rather than just the "traditional" surge each September as a new school year began. I'm not going to dispute the date, but rather the suggestion that it was entirely a Usenet phenomenon. AOL opened the floodgates about the same time as Mosaic became available. Wikipedia puts the availability of Mosaic as April-October 1993, depending on client OS. So two things happened that year: AOL began a steady stream of newbies for the 'net, and the WWW started to gather momentum.
I disagree with those who think Eternal September is over simply because Usenet is now seldom-used. Rather, as society dumbs itself down more and more, Eternal September worsens.